(i) Field of the Invention
The present invention relates to electrolytic in-process dressing grinding, more specifically, to a method and an apparatus for nozzle type ELID grinding with no grindstone-opposed electrode.
(ii) Description of the Related Art
As technology advances, demand of ultraprecision machining is being remarkably sophisticated. As mirror grinding means that meets the demand, an electrolytic in-process dressing grinding method (hereinafter referred to as ELID grinding method) was developed and published by the present applicant (Riken symposium “Advances of New Technology for Mirror Grinding”, held on Mar. 5, 1991).
In the ELID grinding method, as schematically shown in FIG. 1, a conductive grindstone 54 is used in place of an electrode in a conventional electrolytic grinding process, and there is provided an electrode 52 opposed to the grindstone with a gap (hereinafter referred to as grindstone-opposed electrode). A conductive liquid 53 is flowed between the grindstone and the electrode. A voltage is applied between the grindstone 54 and the electrode 52. While the grindstone is dressed by electrolysis, a workpiece is ground by the grindstone. Thus, by carrying out electrolytic dressing of the grindstone simultaneously with the grinding work by using as the anode the metal bonded grindstone 54 and as the cathode the electrode 52 opposed to the surface of the grindstone with a gap, the grinding performance can be maintained and stabilized. In FIG. 1, reference numeral 51 denotes a workpiece (a substance to be ground); 55 does an ELID power source; 56 does a feed element; and 57 does a nozzle for the conductive liquid.
In the ELID grinding method, even in the case of using fine abrasive grains, clogging of the grindstone does not occur because dressing is performed by electrolysis. Thus, by decreasing the abrasive grain size, a very superior work surface, such as a mirror surface, can be obtained by grinding. Therefore, applications of the ELID grinding method to various grinding processes have been proposed as means that can maintain the performance of the grindstone from highly efficient grinding to mirror grinding, and can make a highly precise surface for a short time, which was impossible by any prior art, (e.g., see Patent Documents 1 and 2).
In “Method and Apparatus for Control of Electrolytic Dressing” of Patent Document 1, as shown in FIG. 2, while a conductive liquid is flowed between a grindstone 62 and an electrode 63 from a nozzle 64, a power source 65 and a feed element 66 apply a voltage between the grindstone and the electrode. A workpiece 61 is ground while the grindstone is dressed by electrolysis. In this electrolytic dressing grinding process, a position controller 67 detects a current or voltage between the electrode and the grindstone, and an electrode moving system 68 controls the distance between the grindstone and the electrode such that the detected value is within a set range.
As shown in FIG. 3, “ELID Grinding Apparatus for Fine Shape Machining” of Patent Document 2 comprises a conductive grindstone 72; an X-Y table; an electrolysis electrode 76 provided close to the outer circumferential surface of the grindstone so as to be freely rotatable around the Z axis; and an electrode guide 78. The electrode guide 78 is made up of two contacts one end of each of which is fixed to the electrolysis electrode 76. Each contact extends radially of a circle around the Z axis. The contacts pinch part of a workpiece at positions distant from each other.
[Patent Document 1]
JP-A-7-1333 “Method and Apparatus for Control of Electrolytic Dressing”
[Patent Document 2]
JP-A-2002-1657 “ELID Grinding Apparatus for Fine Shape Machining”
As described above, in the conventional ELID grinding methods, an electrode (grindstone-opposed electrode) is indispensable that is opposed to the outer circumference of the grindstone with a small gap from the grindstone.
Because of this construction, however, if the grindstone is reduced in size, for example, to about 1 to 2 mm in diameter, reduction in size of the electrode and means for setting the electrode may be difficult or impossible. There is a problem that the reduction in size of the apparatus is restricted. Thus, there is a problem that it is difficult to apply the ELID grinding method to, e.g. machining of a micro lens or a mold for the micro lens, needs of which has increased in recent years.